Terahertz parametric amplification of a coherent magnon mode

Parametric amplification is a ubiquitous process in nonlinear systems where a specific parameter can be varied to amplify a coupled degree of freedom. In condensed matter systems, parametric amplification of collective excitations allows one to study non-equilibrium order-parameter physics and potentially control material properties. In this study, we use a pair of intense terahertz (THz) pulses to sequentially excite two distinct coherent magnon modes in an antiferromagnet and find that the lower frequency magnon mode is amplified by driving the higher frequency mode. The nonlinear excitation pathway of this parametric downconversion process is confirmed by two-dimensional THz spectroscopy measurements. Our work provides crucial insights into nonlinear magnonics in antiferromagnets, extending the frontiers of spintronics and magnonics into the ultrafast nonlinear regime.